And when the day of Pentecost (Greek pentekoste, Strong's #G4005) was fully come, they were all with one accord in one place.
And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. (Acts 2:1 - 3, KJV).
On Pentecost God's spirit manifested its presence in the disciples by giving them the power to speak in human languages they did not know (often referred to in Pentecostal circles as speaking in tongues).
A Religious Label
When the label Pentecostal is applied to religious-related groups or customs it usually designates that they believe all Christians should partake of the same manifestations of God's spirit as was displayed in Acts 2. Emphasis is usually placed on experiencing the "gift of tongues," which to some in the movement is speaking unknown or spiritual words that require interpretation in order to understand their meaning.
These people also emphasize the teaching of a "full gospel" or "foursquare gospel." The four squares of such a Gospel revolves around four actions Jesus has or will soon take.
Four Squares
The first square is that Jesus saves according to John 3:16.
For God so loved the world that He gave His only begotten star Son, so that everyone who believes in Him may not perish, but may have everlasting life. (John 3:16, HBFV throughout).
The second "square" is that Jesus baptizes with the Holy Spirit according to Acts 2:4.
And they were all filled with the Holy Spirit; and they began to speak with other languages, as the Spirit gave them the words to proclaim. (Acts 2:4).
The third is he heals bodily according to James 5:15.
And the prayer of faith shall save the sick one, and the Lord will raise him up; and if he has committed sins, they shall be forgiven him. (James 5:15).
The last of the Pentecostal four squares is that Jesus will come again (Second Coming) to receive those who are saved (usually as part of a rapture) according to 1Thessalonians 4:16 - 17.
Because the Lord Himself shall descend from heaven with a shout of command, with the voice of an archangel and with the trumpet of God; and the dead in Christ shall rise first;
Then we who are alive and remain shall be caught up together with them in the clouds for the meeting with the Lord in the air; and so shall we always be with the Lord. (1Thessalonians 4:16 - 17).
Closely Related Terms
The term Charismatic is sometimes used to refer to the same people and behaviors as the term Pentecostal. Many students of the Bible and religious history in general, however, define this group of people as those who developed and came out of the Holy Spirit movement that began around 1900 AD.
Roughly speaking, in the year 2000 around 115 million people worldwide were considered Pentecostal. Some of the major denominations include The Assemblies of God, Church of God in Christ, The Church of God (Cleveland, Tennessee), International Church of the Foursquare Gospel, International Pentecostal Holiness Church and many others.